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  #1  
Old 15 March 2018, 08:45 PM
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Florida New Pedestrian Bridge at FIU Collapses, Killing Multiple People

A pedestrian bridge under construction collapsed Thursday, just days after crews had dropped an elevated 950-ton span in place in a project that was intended to give Florida International University students a safe route across the busy roadway.

The bridge crashed across six lanes of heavily traveled Tamiami Trail, crushing a still undetermined number of cars and killing a still unclear number of people. Police on the scene said at least six people could be dead.

http://www.miamiherald.com/news/loca...205316174.html
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  #2  
Old 16 March 2018, 02:08 PM
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Roll eyes

People are saying "how could they let cars travel beneath an unfinished bridge"? I don't see that they had many choices. I mean, yes, they could close lanes of the freeway during certain stages of the project, but they can't close the entire freeway.

A pedestrian bridge similar (in idea, at least) is to be built across I-5 at Northgate. It's going to be very long, per this design sketch. Ultimately, it will have to cross 8 lanes of the freeway, plus an off-ramp. At least they can cross in stages.

Seaboe
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Old 16 March 2018, 02:43 PM
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Cars travel under unfinished bridges, and bridges that are being repaired/replaced, all the time.
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Old 16 March 2018, 04:15 PM
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The security camera footage I saw on the news this morning will give a structural engineer a good place to look for the cause of the failure since the video seems to show the point of the initial failure.
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  #5  
Old 16 March 2018, 06:19 PM
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Glasses

BoKu posted some pictures on facebook and was discussing the purpose on a particular item. Something about a tensioner.

Seaboe
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  #6  
Old 16 March 2018, 06:29 PM
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Like Lanie said, you travel under unfinished bridges all the time. I currently drive under three or four unfinished bridges on the way to work.

The bridge was being built using a rapid assembly procedure that is designed to minimize how long the road underneath was closed. Six days ago (March 10) this technique and this bridge was considered a marvel;
FIU installs new pedestrian bridge over the Trail in a few hours
http://www.miamiherald.com/news/loca...204506084.html
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  #7  
Old 16 March 2018, 06:48 PM
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Quote:
Originally Posted by Seaboe Muffinchucker View Post
BoKu posted some pictures on facebook and was discussing the purpose on a particular item. Something about a tensioner.Seaboe
An article I read this morning said that they had already determined that the suspension cables where not at the proper tension and where in the process of tightening the tension on the cables when the bridge collapsed.
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Old 16 March 2018, 07:56 PM
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Originally Posted by iskinner View Post
An article I read this morning said that they had already determined that the suspension cables where not at the proper tension and where in the process of tightening the tension on the cables when the bridge collapsed.
So the current question is if that determination was correct or not. If the cables were properly tensioned and the tension was increased is may have gone beyond what the cables could take. Or, the cables were in fact under tensioned and the bridge collapsed before the tension could be corrected.
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  #9  
Old 16 March 2018, 08:35 PM
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As Bodies Are Pulled From Rubble, Questions Mount About Miami Bridge

Quote:
Thursday night, Florida Sen. Marco Rubio tweeted: "The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today."

Because the bridge's central tower and suspension cables were not yet installed, the Miami Herald notes that any cables being tightened were likely wires that ran through the span that fell on Thursday.
Quote:
"Amjad Aref, a professor at University of Buffalo's Department of Civil, Structural and Environmental Engineering, said a suspended bridge is typically built gradually, with the center tower or towers erected early.

"Pictures from the scene of the collapse don't show a central tower.

"'Whoever is going to investigate, they will ask the fundamental question: shouldn't the tower be there, and the cables ready to connect to the structure, when you lift it?' Aref said. 'That's a question for them to answer.'"
Looks like the suspension cables may not have been installed yet. Which some are wondering why not.

Lots and lots of questions to occupy investigators for many weeks I am sure.
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Old 16 March 2018, 10:51 PM
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Quote:
Originally Posted by iskinner View Post
An article I read this morning said that they had already determined that the suspension cables where not at the proper tension and where in the process of tightening the tension on the cables when the bridge collapsed.
I am pretty sure that is a misquote or mischaracterization of what was going on.

In its finished form, this was to be a cable-stayed bridge with a tower and diagonal cables. However, at the time of collapse neither the tower nor stay cables had been installed.

As I understand it, the bridge was specifically designed so that it is strong enough to safely support its own weight without the tower and cables, but does not get its full strength and stiffness until the tower and cables are installed. The innovation there is that it allows the bridge to be built off-site, transported in one piece, and put into place with a minimum of disruption to traffic.

The hydraulic post-tensioning ram that I observed in some photos of the wreckage is almost certainly unrelated to the tower and stay cables. That said, without better photos I'm not in any position to say what it was being used for, or even if it was in use at the time of the collapse. But its presence in the wreckage, especially this early in the construction and before the tower and stay cables are in place, seems a bit odd to me.

Typically, they use machines like that to adjust the tension in steel tendons that run through a structure. I'm not a real engineer, and I don't understand this particular bridge all that well, but I'd speculate that the decision to apply post-tensioning before installing the tower and stay cables might appear in the final report as a causal factor.
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  #11  
Old 17 March 2018, 12:00 AM
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There is a type of construction called post tensioned concrete. The cables are in tubes embedded in the concrete. After the concrete is cured, the cables are pulled tight to put the concrete in compression. Here is a site that explains this much better than I can. https://www.concretenetwork.com/post-tension/
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  #12  
Old 20 March 2018, 01:32 PM
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The NTSB is investigating. Yesterday, they sent out a series of updates on their NTSB_Newsroom Twitter feed. One of them had an engineering plan for the bridge:





I'm no engineer, so I'll leave it to others to make sense of it.

~Psihala
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  #13  
Old 20 March 2018, 03:31 PM
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I would say that Mr. W. Denny Pate is answering more questions than he ever thought possible. As the person who sealed and signed the drawing, he is responsible for the bridge design. However, I can't tell if he is a structural engineer.

ETA: By the way, there are many more drawings than that one.
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  #14  
Old 20 March 2018, 04:20 PM
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The final design might well be fine in itself. It might be the way it was being constructed that was the issue. (The order in which they put parts up, for example).

I gather that the main span was in place but without the "suspension" tower and apparatus (the guy wires) that would hold it up and give it strength when complete. It seems likely that it would be somewhat precarious in that situation, or the suspension apparatus wouldn't be needed. So you'd presumably want that phase of construction to be as short as possible, and not to put undue stress or load on it until the rest was in place. Or find a way to put it up without leaving it in that state at all.
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Old 22 March 2018, 12:58 AM
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A popular photo of the Golden Gate bridge under construction:

link

I actually bought a copy & framed it years ago. Obviously they built the towers, set the huge cables on each side that ran the length of the bridge, then hung the smaller vertical cables and built the roadway sections until they met in the middle.

I read the tower & cables on the Florida bridge were 'for show'; they sure look structural in the drawing posted. But it's virtually impossible to think the reason for the failure is something as simple as "They should've built the tower and set the cables first".
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  #16  
Old 22 March 2018, 02:38 AM
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Quote:
Originally Posted by hoitoider View Post
...I read the tower & cables on the Florida bridge were 'for show'...
That is currently a matter of some debate. There are statements to that effect even in early reports from the NTSB, but other credible reports to the contrary. I'm waiting for the dust to settle on this one.
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  #17  
Old 22 March 2018, 04:12 PM
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The actual complete structural drawings show whether or not the tower and cables are for show or not. My impression is that they are for show but I am an electrical engineer, not a structural engineer. That said, I have worked or the illumination of several bridges some of which have design elements that appear structural but are not.
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  #18  
Old 22 March 2018, 05:25 PM
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Quote:
Originally Posted by RichardM View Post
The actual complete structural drawings show whether or not the tower and cables are for show or not...
I have since encountered detail drawings of the attachments for the "cables," (they're actually 16" diameter hollow tubes), and their rather superficial anchors to the canopy back up the idea that the pylon and "cables" are just for aesthetics, and add relatively little strength or stiffness.

So, mea culpa, I was wrong on this one. I looked at the structure and was completely confident from the arrangement of the truss members that I understood the reasoning behind them. I felt that they had an intrinsic logic that revealed the structure beneath. And I was wrong.

--Bob K.
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  #19  
Old 22 March 2018, 09:06 PM
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I do know that when the story first broke, many of the links I found were to articles only a few weeks before, touting the innovative design that would allow the main span of the bridge to be put in place in a single day. Obviously, that design and process are going to have to undergo some pretty serious re-evaluation.
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  #20  
Old 23 March 2018, 01:40 AM
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Any time there is a structural failure, this will be a rigorous investigation. There will be civil charges filed for sure and very likely criminal charges also. The question, to highly simplify it, is whether the design was inadequate or the construction process did not follow the design. My inclination at this very early stage is that the construction did not follow the design. The bridge had to have sufficient strength to support itself before being moved into place. The post tensioned cables should (I don't know this for a fact) be tight for this level to be reached. The engineer reported that the cables were not tightened to spec after the bridge was set in place. I question why this observation wasn't made before it was allowed to set the bridge in place.
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